1. Field of the Invention
The present invention relates to a bass reproduction speaker apparatus (bass is generally referred to as an audio signal with a frequency of about 200 Hz or less) conducting a motional feedback (MFB). More particularly, the present invention relates to a speaker apparatus for reproducing an audio signal in a deep bass band and an ultra bass band.
2. Description of the Related Art
In recent years, it has been desired that very low frequency audio signals such as a deep bass signal, an ultra bass signal, and the like recorded in a magnetic tape, a disk-shaped data recording medium, etc. are reproduced from a music source or an audio visual (AV) source at a sufficient sound volume and quality in households. In general, bass includes deep bass and ultra bass. In a broad sense, an ultra low frequency is also included in bass. There is no special limit to a band of a bass, deep bass, ultra bass, and an ultra low frequency, and it is variously changed in people and countries. In the present specification, the following definitions are used: bass has a frequency in the range of about 80 to about 200 Hz or in the range of about 100 to 200 Hz; deep bass has a frequency in the range of about 40 to about 80 Hz or in the range of about 50 to about 100 Hz; ultra bass has a frequency in the range of about 20 to about 40 Hz or in the range of about 20 to about 50 Hz; and an ultra low frequency has a frequency of 20 Hz or less. There has been a demand for deep bass reproduction speaker apparatuses which can be combined with stereo reproduction apparatuses or AV reproduction apparatuses and which are capable of reproducing an audio signal, and particularly a voice signal, in a deep bass bend, an ultra bass band, and the like as audio or voice sound with a high sound pressure level, in spite of the relatively small sizes of such speaker apparatuses.
In view of the above, a bass reproduction speaker apparatus, which is obtained by combining a speaker component in which a woofer is provided in a small closed cabinet or a small bass reflex cabinet and an electrical circuit module such as an amplifier for driving the speaker component has generally been used.
It is desired that the speaker component be able to effectively reproduce audio signals with fidelity at frequencies as low as possible in spite of the small size of the speaker component. Moreover, it is desired that the speaker component have a sound pressure level-frequency characteristic in which an audio signal with high frequency is attenuated.
It is known that a band-pass speaker can relatively effectively reproduce an audio signal having a low frequency, in spite of its small size, and attenuate an audio signal with a high frequency, so that the band-pass speaker has a preferred characteristic for reproducing bass audio signals. For example, a band-pass speaker is described in H. Yoshii, "Extreme Low Frequency Sound Reproduction by a Passive Radiator and an Acoustic Transformer, Nippon Onkyo Society Lecture Theses, pp. 281-282 (October, 1978); and Colloms, High Performance Loudspeakers, 4th ed., Pentech Press Limited, pp. 123-126 (1991).
A typical cabinet for such a band-pass speaker is divided into two parts, i.e., a front cavity and a back cavity, by a cavity division member. On the side of the back cavity, a speaker unit is provided on the cavity division member and on the side of the front cavity, a passive radiator is provided in an opening of the cabinet. In most cases, a low-pass filter is provided in front of an amplifier for driving the band-pass speaker.
Operation of the conventional bass reproduction speaker apparatus will be described with reference to an equivalent electrical circuit of a band-pass speaker as shown in FIGS. 11 and 12. Here, the moving system of the speaker unit refers to all of the portions which move in synchronization with the vibration of the speaker unit. More specifically, it refers to a diaphragm and a voice coil.
In FIG. 11, F.sub.d denotes a driving force provided from a voice coil of a magnetic circuit of a speaker unit. The driving force F.sub.d is transmitted to a moving system; an inductor M.sub.d denotes an effective moving mass of the moving system of the speaker unit; a capacitor C.sub.d denotes compliance of suspensions (including a surround and an inner suspension); a resistor R.sub.md denotes a mechanical resistance of the moving system of the speaker unit; a resistor R.sub.ed denotes an electromagnetic damping resistance caused by a reverse eleotromotive force of the magnetic circuit of the speaker unit; a capacitor C.sub.B denotes compliance of the air in the back cavity which is converted in terms of an effective diaphragm area of the speaker unit; a resistor R.sub.B denotes a mechanical resistance of the air in the back cavity which is converted in terms of an effective diaphragm area of the speaker unit; a capacitor C.sub.F denotes compliance of the air in the front cavity which is converted in terms of an effective diaphragm area of the speaker unit; a resistor R.sub.F denotes a mechanical resistance of the air in the front cavity which is converted in terms of an effective diaphragm area of the speaker unit; an inductor M.sub.p denotes an effective moving mass of the moving system of the passive radiator; a resistor R.sub.p denotes a mechanical resistance of the moving system of the passive radiator; a capacitor C.sub.p denotes compliance of the suspensions (including the surround and the inner suspension) of the passive radiator; S.sub.d denotes an effective diaphragm area of the speaker unit; S.sub.p denotes an effective diaphragm area of the passive radiator; current V.sub.d denotes a velocity of the moving system of the speaker unit; and current V.sub.p denotes a velocity of the moving system of the passive radiator.
C.sub.B can be expressed by the following equation: ##EQU1## where, V.sub.B : volume of the back cavity (m.sup.3)
.rho.: air density (Kg/m.sup.3) PA1 C: sound velocity (m/sac) PA1 S.sub.d : effective diaphragm area of the speaker unit (m.sup.2) PA1 R.sub.CB : acoustic mechanical resistance of the air in the back cavity. PA1 k: is a constant
The term V.sub.B /(.rho..times.C.sup.2) is referred to herein as the acoustic compliance. The acoustic compliance of the air in the back cavity changes significantly under the condition of a constant volume of the back cavity when the effective diaphragm area S.sub.d of the speaker unit to be attached is changed.
R.sub.B can be expressed by the following equation: EQU R.sub.B =R.sub.CB .times.k.times.S.sub.d.sup.2
where,
Accordingly, the mechanical resistance R.sub.B of the air in the back cavity also changes in accordance with the square of the effective diaphragm area S.sub.d.sup.2 of the speaker unit. That is, the acoustic compliance and mechanical resistance are converted to compliance and mechanical resistance which act on the diaphragm of the speaker unit.
In FIG. 12, (A) As a sound pressure level-frequency characteristic curve when a motional feedback is not used.
The band-pass speaker has three resonance frequencies. These frequencies are referred to as f.sub.1, f.sub.r, and f.sub.2 in the order of increasing frequency. An impedance-frequency characteristic curve of the band-pass speaker is generally as shown in FIG. 17. The resonance frequency f.sub.1 can be calculated by using a synthetic mass of M.sub.d and M.sub.p, and a synthetic compliance of C.sub.d, C.sub.B, C.sub.F, and C.sub.p. At f.sub.1, the phase of V.sub.d is almost the same as that of V.sub.p. The antiresonant frequency f.sub.r can be calculated by using M.sub.p and a synthetic compliance of C.sub.p and C.sub.F. At f.sub.r, V.sub.d becomes minimum. The resonance frequency f.sub.2 is calculated by using M.sub.d and a synthetic compliance of C.sub.B and C.sub.F. At f.sub.2, the phases of V.sub.d and V.sub.p are shifted by nearly 180.degree.. When the frequency is smaller than f.sub.1 or larger than f.sub.2, a characteristic in which a sound pressure level is attenuated at about 12 dB/oct is obtained.
In general, the following relationships: C.sub.d &gt;C.sub.B, C.sub.d &gt;C.sub.F, and C.sub.p &gt;C.sub.B, C.sub.p &gt;C.sub.F are obtained, i.e., since stiffness (the reciprocal of compliance) of the air in the cabinet is larger than that of the edge and damper of the speaker unit or that of the passive radiator. C.sub.B and C.sub.F are dominant in the resonance frequency, and C.sub.d and C.sub.p can generally be ignored (the resonance frequency is changed a great amount due to the change of the values of C.sub.B and C.sub.F, and the resonance frequency is not changed a great amount due to the change of the values of C.sub.d and C.sub.p). In addition, f.sub.1 is changed in a great amount due to the value of M.sub.p rather than that of M.sub.d. Thus, f.sub.1 is determined by M.sub.p and a synthetic compliance of C.sub.B and C.sub.F ; and f.sub.r is determined by M.sub.p and C.sub.F.
A resonance Q value (relating to the sharpness of resonance) is determined by the magnitude of R.sub.md, R.sub.B, R.sub.F, R.sub.p, and R.sub.ed. In general, since the following relationships: R.sub.ed &gt;R.sub.md, R.sub.ed &gt;R.sub.B, R.sub.ed &gt;R.sub.F, and R.sub.ed &gt;R.sub.p are obtained, the resonance Q is greatly changed by R.sub.ed. Thus, in order to obtain a sound pressure level-frequency characteristic curve having a plateau between f.sub.1 and f.sub.2, the following is conducted. M.sub.d, M.sub.p, C.sub.B, and C.sub.F are set at appropriate values so that the height of each resonance peak f.sub.1 and f.sub.2 is aligned, and R.sub.ed is made sufficiently large so as to lower each resonance peak. Accordingly, a sound pressure level-frequency characteristic curve having a plateau between f.sub.1 and f.sub.2 is obtained. Here, the frequency distance between f.sub.1 and f.sub.2 is at most 1.5 to 2 octaves, and if the distance exceeds this value, a characteristic curve having a concave shape between f.sub.1 end f.sub.2 is obtained.
The resonance Q is in proportion to mass/(compliance.times.resistance), so that as M.sub.d and/or M.sub.p increase and as C.sub.B and/or C.sub.F lower, the resonance Q becomes higher and a greater value of R.sub.ed is required. In the case where R.sub.ed is not large enough, a sound pressure level-frequency characteristic curve (A) having peaks at f.sub.1 and f.sub.2 as shown in FIG. 12 is obtained. R.sub.ed operates as an electromagnetic caused by a reverse electromotive force of the voice coil generated when the moving system of the speaker unit vibrates. Since R.sub.ed =(magnetic flux density of the magnetic circuit.times.effective conductor length of the voice coil).sup.2 /DC resistance of the voice coil, R.sub.ed is generally larger in a speaker unit which has a strong magnetic circuit due to a large magnet.
In order to shift a reproduction frequency band toward an ultra bass band, it is required to lower f.sub.1 and f.sub.2, in particular, f.sub.1 by increasing M.sub.p, M.sub.d, C.sub.B, and C.sub.F. When M.sub.p is increased, the sound pressure level is likely to be totally lowered; however, this does not cause a significant problem since an amplifier with a high power level can easily be realized in recent years. Here, when M.sub.d and M.sub.p alone are increased, the resonance Q becomes higher and peaks are formed in the sound pressure level-frequency characteristic curve, so that it is also required to increase C.sub.B and C.sub.F.
The band-pass speaker uses resonance and has a band-pass characteristic, so that the speaker has relatively high efficiency and is suitable for reproducing a bass. This speaker is driven by an amplifier, whereby a bass reproduction speaker apparatus which reproduces a deep bass is constituted. When the frequency is several hundreds of Hz or more, the characteristic is deteriorated because a standing wave is superimposed on a normal voice signal wave to be reproduced in the cabinet. Thus, in most cases, a low-pass filter is provided to attenuate a signal with a high frequency.
As is described above, in order to shift the reproduction frequency band toward the ultra bass band, it is required to increase M.sub.d, M.sub.p, C.sub.B, C.sub.F, and R.sub.ed. However, there is a limit to the increase in R.sub.ed in view of a size of a magnet of a magnetic circuit and a resultant cost. In addition, since the resonance Q is in proportion to mass/(compliance.times.resistance), it is required to increase C.sub.B and C.sub.F rather than M.sub.d and M.sub.p so as not to cause a resonance peak in the sound pressure level-frequency characteristic curve. C.sub.F is a volume of the front cavity/(air density.times.air sound velocity.sup.2 .times.(effective diaphragm area of the speaker unit S.sub.d.sup.2)). In view of the desire for miniaturization of the bass reproduction speaker apparatus, it is not desired that the cabinet volume be increased so as to increase C.sub.B and C.sub.F. In order to increase C.sub.B and C.sub.F without increasing the cabinet volume, there is no choice but to lower the effective diaphragm area S.sub.d of the speaker unit.
More specifically, in the above-mentioned conventional structure, there is a limit to the increase in R.sub.ed, so that for the purpose of reproducing the ultra bass, there is no choice but to lower the effective diaphragm area S.sub.d of the speaker unit so as not to cause a resonance peak in the sound pressure level-frequency characteristic curve. That is, a diameter of the speaker unit has to be lowered. As a result, the maximum air volume which a diaphragm of the speaker unit can oscillate is lowered and the maximum output sound pressure level of an ultra bass is lowered. Therefore, it can be said that the capability of the speaker unit comes to its limit before the power of the amplifier does.
Accordingly, in the conventional structure, when an ultra bass signal is reproduced with a constant frequency by using a small cabinet, the diameter of the speaker unit has to be lowered. Thus, there are the following problems even though an amplifier with a large output level is easily realized in recent years. A high maximum output sound pressure level cannot be obtained; and it is difficult to realize a speaker unit which can reproduce a bass in spite of its small size, since the magnetic circuit of the speaker unit should be made extremely large.
Moreover, when the effective diaphragm area of the speaker unit is forced to be increased in order to increase the maximum output sound pressure level, C.sub.B and C.sub.F are lowered and it is required to increase M.sub.d and M.sub.p so as not to increase the resonance frequency. As a result, the resonance Q at the above-mentioned two resonance frequencies f.sub.1 and f.sub.2 becomes very high, and high peaks cannot be damped even though R.sub.ed is slightly increased. Thus, a sound pressure level-frequency characteristic curve having a plateau cannot be obtained.